The game's 'coding'.
So as I know which of my icons are being copied I could just replace those with the correct capital letter, and then add the icons to an empty spot?
Can I asume that the sprite/icon that will be copied is static, making this work around viable?
Well it's the E, D, H, some numbers and bits of walls (so far as I know, there may be more) that are being dropped, and there aren't enough blank tiles in the tileset to do that replacement. Thing is, it's not even replacing some tiles with others -- it's actually just ignoring them outright.
Background for this test: every 'blank' tile in the tileset grid had a number drawn in it. That means that all blank spots that are getting drawn with a tile now show up with a number. Blank spots that aren't getting drawn won't. So, for instance, this is the title screen:
The spaces are drawn tiles that were blank. Now they have a (badly-drawn) 6 in them. The areas around the text is just undrawn.
Now we load Arena twice to start the tiledrop bug. It's not just forgetting what tile is what -- it actually
does not draw those tiles.
Thankfully this is easily worked around by restarting DF every time it drops tiles, but it means Arena can't have graphics (as Arena only loads graphics if you start it twice, which is one of the things that causes a tile drop).